Just looking at this:

http://www.best-chem.co.uk/downloads/mo_flo.pdf

This is the stuff we get in Oz, it should be fairly representative of most H2SO4 Drain products

This stuff is a dark brown liquid

Inhibited sulfuric acid?

So it is loaded with Cu and Fe cations to prevent it corroding pipes? Maybe these are the impurities that are so hard to remove...

Just a thought

Blindreaper:

There seems to be quite a bit of discussion about removing the impurities from these products, so i thought this might be of interest

Thats add my mo-flow is a rotten yellow colour. Most high conc. H2SO4 is an yellow/brown colour. It appears to have no impurities and works for all applications I have used it for.

Last thing I have no idea what was the point of your post. You basicly told us that mo-flo has contaminents in it and what it is.

I have some 98% Industrial grade H2SO4 that is a very dark yellow/brown, it looks similar to vanilla essence (sp?).

It has been generally agreed at the forum that drain cleaner H2SO4 is OK for most synths and that the impurities don't cause any serious problems, mostly just discoloration.

Apparently drain-cleaner H2SO4 is made from waste industrial acid, so contaminants could vary depending on what the acid was originally used for.

I'd also like to know how they would stop it from corroding metals?

Drain cleaner grade H2SO4 would be fine for most synthesis, the dyes shoulden't affect the reaction. However if you want to really pure you can try distilling it. It takes time though its boiling point is very high, near the melting point of lead.